Submergence requirement vs NPSH margin for Vertical Pumps (VS4)

[nirlt]

I have to finalize pump model for Vessel mounted Vertical pumps - VS4 type. The vessel has boot with dimensions such that minimum submergence requirement for pump shall be maintained within boot.

The applicable project specification call for NPSH (A-R) of >= 1.0m.

The suction pressure for the pump is 1.1 kg/cm²a and vapour pressure is 0.914 kg/cm²a. The NPSHa at Impeller is 1.4 m whereas the required NPSH for Pump model (under evaluation) at rated point is 1.8m.

As such this margin(0.4m) not acceptable but the minimum submergance requirement for impeller is getting satisfied.

Please advice whether required NPSH margin is to be ensured even when the minimum submergence requirement for the pump impeller is satified ?

 

[TenPenny]

The short answer is that NPSH and submergence are two different things.

If your project specification calls for a particular NPSH margin, then you have to satisfy your project specification.

If you have to feed two children, and you only feed one, you haven't fed them both.